Modern technology has produced a demand for a number of substances which may be transported via pipe lines. This has resulted in the development of technology for constructing pipe lines underground so they will have a minimal ecological impact and will not pose problems for surface transportation systems such as railroads or highways.
A majority of underground pipe lines use metallic pipes with protective outer casings in areas of abnormal mechanical stress such as under road beds. The pipe segments and protective casings in metallic lines are generally constructed of iron or steel and they have a tendency to corrode in the underground environment. During the normal corrosion process, a negative voltage of about 650 millivolts is created. This corrosion can be avoided however if a negative potential of approximately 850 millivolts is applied to the pipe line elements. It is imperative that all of the conductive surfaces of the pipe line be maintained at a negative potential greater than the normal corrosion potential if the corrosion process is to be stopped. To ensure the accurate maintenance of the required potential on the pipe line elements, pipe segments are insulated from each other by insulating members positioned between coupling flanges and protective casings are insulated from the primary pipe line by insulating spacers.
Circuit connecting wires are connected to each side of the coupling flanges and these two electrical connections are used to check the insulation between flanges as well as to take readings of the protective voltages. The wires are usually connected between the pipe flanges and check boxes located above the flanges at the top of the earth surface. Each connection is measured at this point and the measurement is usually against a copper sulfate cell.
If the insulated flanges are shorted together due to a lightning discharge in the immediate vicinity or because of mechanical movement, it is extremely difficult to determine that a short has occurred because the electrical resistance of the individual elements of the pipe is extremely low and the cathodic or protective voltage on adjoining flanges is the same.
The electrical resistance of the metallic pipe is so small that it is virtually impossible, with known measuring apparatus, to accurately determine where adjacent pipes are shorted together. The problem is further complicated, since other pipes and insulators either in series or parallel are interconnected with the pipe under study, thus forming additional short circuits around an insulator of interest. For numerous safety reasons, it is imperative that the insulation between flanges and pipes and casings and pipes remain in a servicable condition and that the insulators be replaced when a breakdown results in shorts between elements. Because the pipe systems are underground, visual inspection of the insulation means is precluded and as previously stated, electrical checking of the insulation is hampered by the identical cathodic voltages applied to the elements.
In addition to corrosion problems encountered in primary undergound supplied pipe lines, serious corrosion problems exist in feeder lines coupling major lines to individual users. For instance, gas meter installations at user destinations are insulated from the gas distribution line to eliminate their direct electrical connection to water or electrical services which are connected to ground thru appliances or thru grounding means. The gas service is connected to the main distribution gas line which is protected by the use of a cathodic voltage of approximately 850 millivolts. Thus if the insulator between the feeder gas line and meter is shorted out, a direct short occurs to the water or electrical ground circuit. This shorts out the 850 millivolt cathodic potential on the gas line. The water line is normally not protected by a cathodic voltage and if it is copper, the normal rust voltage of 650 volts is much lower and this will destroy an iron or steel line fairly rapidly.